Spastic Long-Lasting Reflexes of the Chronic Spinal Rat Studied In Vitro

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Abstract
Over the months following sacral spinal cord transection in adult rats, a pronounced spasticity syndrome emerges in the affected tail musculature, where long-lasting muscle spasms can be evoked by low-threshold afferent stimulation (termed long-lasting reflex). To develop an in vitro preparation to examine the neuronal mechanisms underlying spasticity, we removed the whole sacrocaudal spinal cord of these spastic chronic spinal rats (>1 mo after S2 sacral spinal transection) and maintained it in artificial cerebral spinal fluid in a recording chamber. The ventral roots were mounted on monopolar recording electrodes in grease, and the reflex responses to dorsal root stimulation were recorded and compared with the reflexes seen in the awake chronic spinal rat. When the dorsal roots were stimulated with a single pulse, a long-lasting reflex occurred in the ventral roots, with identical characteristics to the long-lasting reflex in the awake spastic rat tail. The reflex response was low threshold ( T), short latency, long duration (∼2 s), and enhanced by repeated stimulation. Brief high-frequency stimulation trains (0.5 s, 100 Hz, 1.5 × T) evoked even longer duration responses (5–10 s), with repeated bursts of activity that were similar to the repeated muscle spasms evoked in awake rats with stimulation trains or manual skin stimulation. Stimulation of a given dorsal root evoked long-lasting reflexes in both the ipsilateral and contralateral ventral roots. Long-lasting reflexes did not occur in the sacrocaudal spinal cord of acute spinal rats (S2 transection), which is similar to the areflexia seen in awake acute spinal rats. However, long-lasting reflexes could be made to occur in the acute spinal rat by altering K+ (7 mM) or Mg2+ (0 mM) concentrations, or by application of high doses of the neuromodulators norepinephrine (NE, >20 μM) or serotonin (5-HT, >20 μM). In chronic spinal rats, much lower doses of these neuromodulators (0.1 μM) enhanced the long-lasting reflexes, suggesting a denervation supersensitivity to 5-HT and NE following injury. Higher doses of NE or 5-HT produced a paradoxical inhibition of the long-lasting reflexes. The high dose inhibition by NE was mimicked by the α2-adrenergic receptor agonist clonidine but not the α1-adrenergic receptor agonist methoxamine. In summary, the sacral spinal in vitro preparation offers a new approach to the study of spinal cord injury and analysis of antispastic drugs.